Protective relay system provided with difference and addition filters

ABSTRACT

A protective relay system detects a first electric variable and a second electric variable of a power system to discriminate in a time series whether or not a fault portion of a line or equipment included in the power system exists within a predetermined range, so as to determine whether protection of the power system should be carried out. The system includes a digital filter for outputting a first difference electric variable data indicative of a difference between at least two sample data of plural sampling data of the first electric variable and a difference between at least two sample data of plural sample data of the second electric variable at plural sampling times of the time series. The system also includes an addition filter for outputting first and second additive electric variable data indicative of orthogonal vector data with respect to the first and second difference electric variable data. The system further includes a relay control unit for calculating controlled variables of a relay operation in the power system on the basis of the first and second difference electric variable data at a certain sampling time, so as to judge whether or not protection of the power system should be carried out.

TECHNICAL FIELD

This invention relates to a protective relay system provided with adifference filter and an addition filter, and more particularly to aprotective relay system substantially free from influence of harmoniccomponents which may be included in a fault current.

Background Art

The main technical object of protective relay systems used forprotecting the power system is to lessen influence of harmoniccomponents of fault current and fault voltage produced at the time ofsystem failure. Particularly, in recent years, since charge capacitycomponent of the system in equipments or facilities such as cable(power) transmission line and/or phase modifying capacitor, etc. hasbeen increased, the order of harmonic wave produced in the system powerhas tendency to increase (value twice or three times greater than thefundamental wave).

For this reason, in a method of attenuating harmonic components byfilter which has been conventionally applied, it is necessary to prolongdelay time of the filter in order to ensure desired attenuationquantity, resulting in delayed operation time of the relay. In order tosolve such a problem, there has been conventionally employed anapproximation system such that even if any harmonic component isincluded in the system power, there essentially results no influence ofthe harmonic wave.

One example of the system employed at present will be described below.

In a transmission line 2 of FIG. 1, assuming now that voltage andcurrent at point A of installation of a protective relay 1 arerespectively v and i when transmission line impedance constants up tothe fault point F are such that resistance is R and inductance is L, thedifferential equation of the transmission line 2 is expressed by thefollowing equation (1) in the case where fault point (F) voltage iszero. By carrying out approximate calculation of the differential term(di/dt) of this equation (1), it is possible to calculate, with highaccuracy, value proportional to inductance L without eliminatingharmonic wave by using filter. ##EQU1##

An example of an actual method for digital operation practically appliedis indicated below.

    v.sub.m +v.sub.m-1 =R·(i.sub.m +i.sub.m-1)+L·(i.sub.m -i.sub.m-1) v.sub.m-1 +v.sub.m-2 =R·(i.sub.m-1 +v.sub.m-2)+L·(i.sub.m-1 -V.sub.m-2)             (2)

When X(=ω₀ ·L) is calculated from the equation (2), the followingequation (3) is obtained. The frequency characteristic of the reactancevalue X_(m) /X (true value) is represented by the following equation(4), and its characteristic is as indicated by line (a) of FIG. 2. FIG.2 is a graph in which frequency is taken on the abscissa and reactancemeasured value is taken on the ordinate. In this case, the line (a)indicates the characteristic in the case where sampling frequency is 600Hz, and the line (b) indicates the characteristic in the case wheresampling frequency is 4800 Hz. ##EQU2##

    L.sub.m /L (true value)=tan (ω.sub.0 T/2)/tan (ωT/2) (4)

In this case, since i_(m) =I·sin (ωt_(m)) and v_(m) =V·sin (ωt_(m) +θ),differential approximation quantity "i_(m) -i_(m-1) " and diffentiatedquantity "v_(v) +v_(m-1) " can be respectively expressed by thefollowing equations (5) and (6).

    i.sub.m -i.sub.m-1 =2I·sin (ωT/2)·cos(ωt.sub.m -ωT/2)     (5)

    v.sub.m +v.sub.m-1 =2V·cos(ωT/2)·sin(ωt.sub.m -ωT/2+θ)                                      (6)

In the above equation, T is sampling period, ω is angular frequency, andθ is voltage lead phase with respect to current.

As shown in FIG. 2, it is understood that according as frequencydeviates from that of the fundamental wave, value of L_(m) /L (truevalue) becomes smaller than 1. This value is permitted to keep value inthe vicinity of 1 at values approximately twice or third times greaterthan the fundamental wave if the value of (ωT/2) is further held down tolower value (the sampling period is reduced). The frequencycharacteristic when the sampling frequency is set to value eight timesgreater than the fundamental wave is shown FIG. 2(b). Namely, from aqualitative point of view, the relationship between the differentialapproximation quantity "i_(m) -i_(m-1) " and the differentiated quantity"v_(m) +v_(m-1) " are as indicated by the following equations (7) and(8). From these equations, it is indicated that its approximationaccuracy is improved.

    sin (ωT/2)≈ωT/2, cos(ωT/2)≈1 i.sub.m -i.sub.m-1 =2I·sin (ωT/2)·cos (ωt.sub.m -ωT/2) ≈2I·(ωT/2)·cos (ωt.sub.m -ωT/2)                              (7)

    v.sub.m +v.sub.m-1 =2V·cos (ωT/2)·sin (·t.sub.m -ωT/2+θ) ≈2V·sin (ωt.sub.m -ωT/2+θ)                      (8)

Accordingly, if the sampling frequency is caused to become high (theperiod is reduced), the approximation accuracy of differentiation can beimproved. However, the value of the equation (7) becomes very smallvalue with respect to the amplitude value I, and noise error ε includedin the sample or sampled data "i_(m), i_(m-1) " effectively becomesgreat. From a viewpoint of practical use, it is very difficult to ensureaccuracy.

    (i.sub.m -i.sub.m-1)/(ω.sub.0 T/2) ≈2I·(ω/ω.sub.0)·cos (ωt.sub.m -ωT/2))+ε/(ω.sub.0 T/2)               (9)

In the above equation, ε is noise error. This noise error is whitenoise, etc. produced in the analog circuit, and quantization error, etc.produced at the time of analog-to-digital conversion.

When the sampling period T=i/4800 sec., and ω₀ =2π·50 Hz, error of theportion "ε/(ω₀ T/2)" of the equation (9) is as indicated by thefollowing equation (10). The error is amplified so that it becomes equalto value 30 times greater than the original value.

    ε/(ω.sub.0 T/2)=96ε/π≈30.557ε(10)

SUMMARY OF THE INVENTION

This invention has been made in view of the above-mentionedcircumstances, and its object is to provide a protective relay system inwhich error amplification by differential approximation is suppressed sothat there results the characteristic that L_(m) /L (true value) isunlimitedly equal to 1 in broad frequency band, and free from influenceof harmonic components produced in fault voltage/fault current of thepower system.

To achieve the above-mentioned object, a protective relay systemaccording to this invention is directed to a protective relay systemadapted for detecting, in time series manner, first electric variable(quantity of electricity) and second electric variable of the powersystem to discriminate on the basis of changes in the respectiveelectric variables in the time series whether or not fault portion(point) of line or equipment included in the power system exists (falls)within a predetermined range thus to protect the power system, thesystem comprising: digital filter means including a difference filterfor outputting first difference electric variable data indicative ofdifference between at least two sample data of plural sample data of thefirst electric variable and second difference electric variable dataindicative of difference between at least two sample data of pluralsample data of the second electric variable at plural sampling times ofthe time series, and an addition filter for outputting first and secondadditive electric variable data indicative of data respectivelyorthogonal to the first and second difference electric variable data interms of vector; and relay control means for calculating controlledvariable of the relay operation in the power system on the basis of thefirst and second difference electric variable data at a certain samplingtime, first and second additive electric variable data at the certainsampling time point, the first and second difference electric variabledata at any other sampling time point and the first and second additiveelectric variable data at the other sampling time point to judge(discriminate), on the basis of the controlled variables, the operationas to whether or not protection of the power system should be carriedout.

In the above-mentioned protective relay system, the difference filter isa filter in which transfer function "Z^(-k) =Z^(-q) " (Z is Z transformoperator and k<q) is used to output voltage data v_(jm) and current datai_(jm) serving as the first and second difference electric variable dataat certain sampling time t_(m) and voltage data v_(jm-p) and currentdata i_(jm-p) at any other sampling time t_(m-p), and the additionfilter is a filter in which transfer function "(1+Z⁻¹ +Z⁻² +. . .+Z^(-n)) (1+Z⁻¹)" ("n+1=k+q") is used to output voltage data v_(sm) andcurrent data i_(sm) serving as the first and second additive electricvariable data at the certain sampling time t_(m) and voltage datav_(sm-p) and current data i_(sm-p) at the other sampling time t_(m-p).

Further, in the above-mentioned protective relay system, the relaycontrol means includes controlled variable calculation means forcalculating relay controlled variable including at least one ofreactance value, ohm value and operation/suppression quantity on thebasis of the voltage data v_(jm) and v_(jm-p) and the current datai_(jm) and i_(jm-p) which are outputs of the difference filter of thedigital filter means and the voltage data v_(sm) and v_(sm-p) and thecurrent data i_(sm) and i_(sm-p) which are outputs of the additionfilter; and operation judgment means for judging whether or not value ofthe relay controlled variable calculated by the controlled variablecalculation means has a predetermined relationship with respect to apredetermined setting value and constant. These components are thefundamental configuration of the protective relay system according tothis invention.

In accordance with the protective relay system according to the firstaspect of this invention, in the above-mentioned fundamentalconfiguration, the controlled variable calculation means is constitutedby reactance value calculation means for determining reactance valueX_(m) by the following equation (A) on the basis of the current datai_(jm) and i_(jm-p) which are outputs of the difference filter, and thevoltage data v_(sm) and v_(sm-p) and the current data i_(sm) andi_(sm-p) which are outputs of the addition filter ##EQU3##

Moreover, the operation judgment means judges the discriminant "X_(m)≦X_(s) " from reactance value X_(m) and setting value X_(s) determinedby the reactance calculation means.

In accordance with the protective relay system according to the secondaspect of this invention, in the above-mentioned configuration, thecontrolled variable calculation means is constituted byoperation/suppression quantity calculation means for calculatingoperation/suppression quantities a_(m) and b_(m) corresponding toreactance value by the following equation (B) on the basis of thecurrent data i_(jm) and i_(jm-p) which are outputs of the differencefilter and the voltage data v_(sm) and v_(sm-p) and the current datai_(sm) and i_(sm-p) which are outputs of the addition filter:

    a.sub.m =-v.sub.sm ·i.sub.sm-p +i.sub.sm ·v.sub.sm-p b.sub.m =-i.sub.jm ·i.sub.sm-p +i.sub.jm-p ·i.sub.sm (B)

Moreover, the operation judgment means judges discriminant "b_(m) ·X_(s)-a_(m) ≧KO" from the operation/suppression quantities am and bmdetermined by the operation/suppression quantity calculation means, andsetting value X_(s) and constant KO.

In accordance with the protective relay system according to the thirdaspect of this invention, in the above-mentioned fundamentalconfiguration, the controlled variable calculation means is constitutedby ohm value calculation means for determining ohm value Rm by thefollowing equation (C) on the basis of the current data i_(jm) andi_(jm-p) which are outputs of the difference filter and the voltage datav_(sm) and v_(sm-p) and the current data i_(sm) and i_(sm-p) which areoutputs of the additive filter: ##EQU4##

Moreover, the operation judgment means judges discriminant "R_(m) ≦R_(s)" from the ohm value R_(m) determined by the ohm value calculation meansand setting value R_(s).

In accordance with the protective relay system according to the fourthaspect of this invention, in the fundamental configuration, thecontrolled variable calculation means is constituted byoperation/suppression quantity calculation means corresponding to ohmvalue by the following equation (D) on the basis of the current datai_(jm) and i_(jm-p) which are outputs of the difference filter and thevoltage data v_(sm) and v_(sm-p) and the current data i_(sm) andi_(sm-p) which are outputs of the addition filter:

    c.sub.m =-i.sub.jm ·v.sub.sm-p +v.sub.sm ·i.sub.jm-p b.sub.m =-i.sub.jm ·i.sub.sm-p +i.sub.jm-p ·i.sub.sm (D)

Moreover, the operation judgment means judges discriminant "b_(m) ·R_(s)-c_(m) ≦Kl" from the operation/suppression quantity C_(m) and b_(m)determined by the operation/suppression quantity calculation means, andsetting value R_(s) and constant Kl.

In accordance with the protective relay system according to the fifthaspect of this invention, in the above-mentioned configuration, thecontrolled variable calculation means is constituted by reactance valuecalculation means for determining reactance value X_(m) by the followingequation (A) on the basis of the current data i_(jm) and i_(jm-p) whichare outputs of the difference filter, and the voltage data v_(sm) andv_(sm-p) and the current data i_(sm) and i_(sm-p) which are outputs ofthe addition filter ##EQU5## and ohm value calculation means fordetermining ohm value K_(m) by the following equation (C) on the basisof the current data i_(jm) and i_(jm-p) which are outputs of thedifference filter and the voltage data v_(sm) and v_(sm-p) and thecurrent data i_(sm) and i_(sm-p) which are outputs of the additionfilter: ##EQU6## Moreover, the operation judgment means serves to judge(discriminate) the discriminant (E) on the basis of the reactance valueR_(m) determined by the reactance value calculation means the ohm valueR_(m) determined by the ohm value calculation means:

    (R.sub.m -R.sub.0)·(R.sub.m -R.sub.F)·(X.sub.m -X.sub.0)·(X.sub.m -X.sub.f)≦0            (E)

where

R₀ (ohmic component) is the offset mho near side setting value

X₀ (reactance component) is offset mho near side setting value

R_(F) (ohmic component) is the offset mho far side setting value, and

X_(F) (reactance component) is the offset mho far side setting value

In accordance with the protective relay system according to the sixthaspect of this invention, in the above-mentioned fundamentalconfiguration, the controlled variable calculation means is constitutedby polarity voltage preparation means for preparing polarity voltagev_(pjm) having a predetermined relationship with respect to the voltagedata v_(jm) and/or v_(sm) on the basis of the voltage data v_(jm) whichis output of the differential filter and voltage data v_(sm) which isoutput of the addition filter, and the operation judgment means servesto judge, whether or not the following discriminant holds, on the basisof voltage data v_(jm) and current data i_(jm) which are outputs of thedifference filter, voltage data v_(sm) and current data i_(sm) which areoutputs of the additive filter, polarity voltage v_(pjm) which is outputof the polarity voltage preparation means, and setting values R_(s) andX_(s) :

    v.sub.pjm-p ·{(R.sub.s ·i.sub.sm +X.sub.s ·i.sub.jm)-v.sub.sm } -v.sub.pjm ·{(R.sub.s ·i.sub.sm-p +X.sub.s ·i.sub.jm-p)-v.sub.sm-p }≧K2 (F)

In accordance with the protective relay system according the seventhaspect of this invention, in the above-mentioned sixth aspect, thepolarity voltage preparation means synthesizes a voltage orthogonal in afundamental wave with respect to output v_(sm) of the addition filter interms of vector to output it as the polarity voltage v_(pjm) to theoperation judgment means.

In accordance with the protective relay system according to the eighthaspect of this invention, in the above-mentioned sixth aspect, thepolarity voltage preparation means serves to output a voltage earlier bypredetermined time of voltage data v_(jm) from the difference filter atthe sampling time point t_(m) to the operation judgement means as thepolarity voltage v_(pjm).

In accordance with the protective relay system according to the ninthaspect of this invention, in the above-mentioned sixth aspect, thepolarity voltage preparation means serves to synthesize positive phasevoltage from output v_(jm) of the difference filter and output v_(sm) ofthe addition filter with three-phase voltage of the power system at thesampling time tm being as reference to output it as the polarity voltagej_(pjm) to the operation determination means.

In accordance with the protective relay system according to the tenthaspect of this invention, in the above-mentioned fundamentalconfiguration, the controlled variable calculation means is composed ofcharge current compensation means for determining a predeterminedelectric variable (i_(sm) -C_(s) ·v_(jm)) on the basis of the voltagedata v_(jm) as output of the difference filter, and the current datai_(sm) as output of the addition filter at the sampling time pointt_(m), and setting value C_(s) ; and transmitting/receiving meansadapted for transmitting the predetermined electric variable (i_(sm)-C_(s) ·v_(jm)) delivered from the charge current compensation means toa destination electric station (B electric station) where the protectiverelay system is installed and for receiving electric variable (i_(sm)-C_(s) ·v_(jm)) B from any other protective relay system installed inthe destination electric station (B electric station), and the operationjudgment means serves to judges, on the basis of the predeterminedelectric variable (i_(sm) -C_(s) ·v_(jm)) delivered from the chargecurrent compensation means and the predetermined electric variable(i_(sm) -C_(s) ·v_(jm)) B delivered from the transmitting/receivingmeans, whether or not the following discriminant (G) holds:

    ∥(i.sub.sm -C.sub.s ·v.sub.jm)+(i.sub.sm -C.sub.s ·v.sub.jm)B∥≧ka·{∥i.sub.sm -C.sub.s ·v.sub.jm ∥+∥(i.sub.sm -C.sub.s ·v.sub.jm)B∥}+kb                        (G)

where

∥am∥ is quantity proportional to amplitude value of a.c. electricvariable a at the time point of t_(m),

ka is No. ratio suppression digits (absolute number), and

kb is minimum sensitively current.

The fundamental operation of the protective relay system constructed asabove will be described below.

When sample data of current i=I·sin(ωt) is inputted to the digitalfilter means, processing as described below is carried out.

Initially, by allowing such sample data to be passed through the digitalfilter of the first stage (1+Z⁻¹ +Z⁻² +. . . +Z^(-n)), current i'_(sm)at time point tm is obtained. ##EQU7##

Further, by allowing such sampling data to be passed through digitalfilter of the succeeding stage (1+Z⁻¹), current i'_(sm) at the timepoint t_(m) is as indicated by the following equation. ##EQU8##

Also with respect to the voltage, expansion can be similarly made. Whenthe current i=I·sin(ωt) is caused to be passed through the differencefilter, current i_(jm) at time point tm is as indicated by followingequation (14). ##EQU9##

Also with respect to the voltage, expansion can be similarly made. Inthis case, if the equations (12) and (13) have the relationship thatthey are orthogonal to each other in terms of vector, k+q=n+1 holds.

Moreover, if value as close as to 1 is selected in the fundamental waveso that quantity which determines magnitudes of the right side of theequation (14) does not become |sin((q-k)ωT/2|<<1, performance of thefrequency characteristic can be ensured in the state where noise erroris not amplified. For example, let consider the case where k=0 andq=n+1. At this time, the equation (14) is expressed as follows.

    i.sub.jm =2I·sin ((n+1)ωT/2)·cos (ωt.sub.m -(n+1)ωT/2)                                         (15)

Further, substitution of the relationship of the equation (15) and theequation (13) into the equation gives the following equation. ##EQU10##

Accordingly, if n is caused to be sufficiently greater value, influenceof noise error on the current data of the equation (15) can be lessened,and the frequency characteristic performance with respect to thefundamental wave of X_(m) can satisfy the characteristic of FIG. 2.X_(m) /X (value in terms of the fundamental wave)=tan (ω₀ T/2)/tan(ωT/2).

As described above, in accordance with this invention, even if harmoniccomponents are superimposed on fault current/voltage produced at thetimes of failure of the power system, input data is caused to be passedthrough predetermined digital filters of which characteristics areorthogonal to each other in terms of vector in a broad frequency band,thereby making it possible to approximately solve, with high accuracy, apredetermined differential equation. Thus, high accuracy protectiverelay system can be realized.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a block circuit diagram showing power system to which aprotective relay system according to this invention is applied, FIG. 2is a characteristic diagram showing an example of frequencycharacteristic of reactance measured value for explaining protectiveoperation in protective relay system according to this invention;

FIG. 3 is a block diagram showing the fundamental configuration of theprotective relay system according to this invention;

FIG. 4 is a block diagram showing the configuration of the protectiverelay system according to a first embodiment (first aspect) of thisinvention;

FIG. 5 is a block diagram showing the configuration of the protectiverelay system according to a second embodiment (second aspect) of thisinvention;

FIG. 6 is a characteristic diagram showing reactance characteristic onthe impedance plane in the protective relay system according to thefirst and second embodiments of this invention;

FIG. 7 is a block diagram showing the configuration of the protectiverelay system according to the third embodiment (third aspect) of thisinvention;

FIG. 8 is a block diagram showing the configuration of the protectiverelay system according to the fourth embodiment (fourth aspect) of thisinvention;

FIG. 9 is a characteristic diagram showing ohm characteristic on theimpedance plane in the protective relay system according to the fourthembodiment of this invention;

FIG. 10 is a block diagram showing the configuration of the protectiverelay system according to a fifth embodiment (fifth aspect) of thisinvention;

FIG. 11 is a characteristic diagram showing offset mho characteristic onthe impedance plane in the protective relay system according to thefifth embodiment of this invention;

FIG. 12 is a block diagram showing the protective relay system accordingto the sixth embodiment (seventh aspect) of this invention;

FIG. 13 is a characteristic diagram showing the relationship between themaximum sensitivity angle and setting impedance of the mhocharacteristic in the protective relay system according to the sixthembodiment of this invention;

FIG. 14 is a characteristic diagram showing mho characteristic in whichthe relationship shown in FIG. 15 is represented by current reference.

FIG. 15 is a block diagram showing the configuration of the protectiverelay system according to the seventh embodiment (eighth aspect) of thisinvention;

FIG. 16 is a block diagram showing the configuration of the protectiverelay system according to an eighth embodiment (ninth aspect) of thisinvention;

FIG. 17 is a block diagram sowing the configuration of the protectiverelay system according to a ninth embodiment (tenth aspect) of thisinvention; and

FIG. 18 is a circuit diagram of power transmission line for explainingtransmission equation with respect to the charge current compensation inthe protective relay system shown in FIG. 17.

BEST MODE FOR CARRYING OUT INVENTION

Preferred embodiments of a protective relay system according to thisinvention will now be described in detail with reference to the attacheddrawings.

Prior to description of the embodiments of the protective relay systemaccording to this invention, the fundamental concept of this inventionwill now be described with reference to FIG. 3.

FIG. 3 is a block diagram showing the entirety of a power systemprovided in the protective relay system. In FIG. 3, the protective relaysystem 1 is connected to a transmission line 2 comprising an a.c. powersupply 3, a potential transformer (PT) 4 and a current transformer (CT)5. The potential transformer 4 is voltage transformer used for measuringhigh voltage to measure voltage at a certain point of the transmissionline 2. The current transformer 5 is an instrument transformer tomeasure a current at a certain point of the transmission line 2. Thevoltage and the current are the above-described first and secondelectric variables.

The protective relay system 1 comprises analog processing means 10 forprocessing analog data relating to voltage and current measured atplural sampling points continuous in time series manner, and digitalprotective relay means 15 for carrying out protective relay operation onthe basis of digital data obtained by allowing the analog data toundergo analog-digital conversion.

The analog processing means 10 comprises a voltage sample-hold circuit11 for detecting, every predetermined sampling time, voltage at acertain point of the transmission line 2 measured by the potentialtransformer 4, a current sample-hold circuit 12 for detecting, everysampling time, current at a certain point of the transmission line 2measured by the current transformer 5, a multiplexer 13 for multiplexingor selecting voltage data and current data of time series respectivelyoutputted from the voltage sample-hold circuit 11 and the currentsample-hold circuit 12 to output them, and an analog-to-digitalconverter 14 for converting voltage data and current data delivered fromthe multiplexer 13 from analog signal to digital signal to deliver it tothe digital protective relay system 15.

The digital protective relay system 15 comprises a memory 16 fortemporarily storing respective digital data relating to voltage andcurrent delivered from the analog/digital converter 14 of the analogprocessing means 10, digital filter means 20 for allowing voltage andcurrent digital data as first and second electric variables deliveredfrom the memory 16 to respectively undergo filtering (processing) toobtain output necessary for control, and relay control means 30 fordetermining controlled variable of the relay operation on the basis ofplural outputs from the digital filter means 20.

The digital filter means 20 comprises a difference filter 21 foroutputting first difference electric variable data which is differencebetween at least two sample data of plural sample data with respect tovoltage as the first electric variable and second difference electricvariable data which is difference between at least two sample data ofplural data as the second electric variable at plural sampling times ofthe time series, and an addition filter 22 for outputting first andsecond additive electric variable data which are data respectivelyorthogonal to the first and second difference electric variable data interms of vector. In more practical sense, the differential filter 21 isa filter in which transfer function "Z^(-k) -Z^(-q) " (Z is Z transformoperator, and k<q) is used to output voltage data v_(jm) and currentdata i_(jm) which serve as the first and second difference electricvariable data at a certain sampling time t_(m), and voltage datav_(jm-p) and current data i_(jm-p) at any other sampling time t_(m-p).Moreover, the addition filter 22 is a filter in which transfer function"(1+Z⁻¹ +Z⁻² +. . . +Z^(-n)) (1+Z⁻¹)" ("n+1=k+q") is used to outputvoltage data v_(sm) and current data i_(sm) serving as the first andsecond additive electric variable data at the certain sampling timet_(m), and voltage data v_(sm-p) and current data i_(sm-p) at the othersampling time point t_(m-p).

Further, the relay control means 30 comprises controlled variablecalculation means 31 for calculating relay controlled variable includingat least one of reactance value, ohm value, operation/suppressionquantity on the basis of the voltage data v_(jm) and V_(jm-p) and thecurrent data i_(jm) and i_(jm-p) outputs of the difference filter of thedigital filter means, and the voltage data v_(sm) and v_(sm-p) and thecurrent data i_(sm) and i_(sm-p) which are outputs of the additionfilter, and operation judgment means 32 for judging whether or not thevalue of the relay controlled variable calculated by the controlledvariable calculating means has a predetermined relationship with respectto predetermined setting value and constant. By such a configuration,the relay control means calculates controlled variable of the relayoperation in the power system on the basis of the first and seconddifference electric variable data at a certain sampling time, first andsecond additive electric variable data at the certain sampling time,first and second difference electric variable data at any other samplingtime, and first and second additive electric variable data at the othersampling time to judge, on the basis of the controlled variable, theoperation as to whether or not protection of the power system should becarried out, thus making it possible to control the relay operation ofthe power system.

The gist of this invention resides in that there is used digital filtermeans comprising difference filter 21 using the transfer function"(Z^(-k) -Z^(-q))" and addition filter 22 operative while correlatingwith the operational action of the difference filter 21, and using thetransfer function "(1+Z⁻¹ +Z⁻² +. . . +Z^(-n)) (1+Z⁻¹)". The first toninth embodiments (first to tenth aspects) are characterized in that thekinds of controlled variables calculated by using respective outputs ofthe filters 21, 22 constituting the digital filter means arerespectively different in various manners.

Explanation will now be given in detail in connection with digitalprotective relay means by different configurations caused tocorresponding to different kinds of controlled variables, e.g.,reactance value, ohm value, respective operation/suppression quantities,polarity voltage, and charge current compensation quantity, etc.

FIG. 4 is a block diagram of the first embodiment for explaining theprotective relay system according to the first aspect of this inventionThis invention is characterized in that a measure is taken such thateven if the sampling period is reduced, noise error included in data ofdifferential quantity is not amplified, thereby making it possible toensure performance of the frequency characteristic indicated by theequation (14). In FIG. 4, reference numeral 21 denotes a digitaldifference filter for extracting predetermined frequency components ofvoltage and current of power system (not shown) subject to protection,and reference numeral 22 denotes a digital addition filter forextracting voltage and current orthogonal to output data of thedifference filter 21 in terms of vector even with respect to allfrequency components. Moreover, reference numeral 31 denotes reactancevalue calculation means for calculating reactance, and reference numeral41 denotes operation judgment means for judgment of operation.

Sample values v_(m), i_(m) at time t_(m) of voltage v and current i ofthe power system are inputted to the addition filter 22 of FIG. 4 toallow them to be passed through a filter having transfer function (1+Z⁻¹+Z⁻² +. . . +Z^(-n)) (1+Z⁻¹) (Z indicates Z transform operator) tothereby obtain voltage v_(sm), current i_(sm). Further, the samplevalues v_(m), i_(m) are inputted to the difference filter 21 to allowthem to be passed through a filter having transfer function (Z^(-k)-Z^(-q)) (d+q=n+1; k<q) to thereby voltage v_(jm), current i_(jm).

The reactance value calculation means 31 calculates reactance valueX_(m) on the basis of the following equation (11) from voltage v_(sm)and current i_(sm) obtained at the addition filter 22 and voltage v_(jm)and current i_(jm) obtained at the differential filter at time t_(m),and voltage v_(sm-p), current i_(sm-p) obtained from the addition filter22 and voltage v_(jm-p) and current i_(jm-p) obtained from thedifferential filter 21 at time t_(m-p). Further, the operation judgmentmeans 41 judges, from the reactance value X_(m) obtained at thereactance value calculation means 31 and setting value X_(s), whether ornot X_(m) ≦X_(s) hold to conduct judgment of operation such that if theabove relationship holds, the protective relay is in operative state,while it does not hold, the it is inoperative state. ##EQU11##

Functions of respective digital filters are represented by using Ztransform operator in a manner as indicated by the following equation.It is to be noted that since orthogonal relationship is described indetail in the Disclosure of the Invention, explanation thereof isomitted here (there holds the relationship that the addition filter 22is lag by 90 degrees with respect to the difference filter 21).

(1+Z⁻¹ +Z⁻² +. . . +Z^(-n)) (1+Z⁻¹)

(Z^(-k) -Z^(-q)) (In this case, k+q=n+1)

Further, in the case where setting is made such that k=0, q=n+1 in thedifference filter 21, the transfer function of the addition filter 22may be caused to be (1+Z⁻¹ +Z⁻² +. . . +Z^(-n)) (1-Z⁻¹), (=1-Z^(-n-1)).Moreover, it is apparent that this configuration and the configurationof the addition filter 22 can be realized by divisional configurationconsisting of three digital filters. Namely, input voltage and currentare caused to be first passed through the first filter of the followingequation to input its output to two filters, i.e., a second filter and athird filter, thus making it possible to obtain an output equivalent tothe differential filter and the additive filter 22.

First filter: (1+Z⁻¹ +Z⁻² +. . . +A^(-n))

Second filter: (1+Z⁻¹)

Third filter: (1-Z⁻¹)

The reactance value calculation means 31 of FIG. 4 is means forcalculating reactance value from the protective relay installation pointup to fault point of the transmission line of FIG. 1 by the equation(11). When input voltage and current of the differential filter 21 andthe addition filter 22 are expressed as i=I·sin (ωt), v=V·sin (ωt+θ),the equation (11) can be represented by the equation (16).

The operation judgment means 41 makes a correction as indicated by X_(m)≦X_(s) /tan (ω₀ ·T/2) by X_(m) calculated at the reactance valuecalculation means 31, setting value X_(s) and constant tan (ω₀ T/2) atthe fundamental wave determined in advance to judge whether or not theprotective relay is operative. It is to be noted that, in thedescription of the gist of this invention, description of pluralcollating operations of the operation judgments is omitted here.

The protective relay system of the second embodiment corresponding tothe second aspect will now be described.

FIG. 5 is a block diagram showing the configuration of the protectiverelay system according to the second embodiment of this invention.

In the second embodiment, in place of the reactance value calculationmeans 41 of FIG. 4, reactance value operation/suppression quantitycalculation means 42 for calculating the following equation (17) isapplied.

    a.sub.m =-v.sub.sm ·i.sub.sm-p +i.sub.sm ·v.sub.sm-p b.sub.m =-i.sub.jm ·i.sub.sm-p +i.sub.sm-p ·i.sub.sm (17)

When current input and voltage input of the digital filter 20 consistingof difference filter 21 and addition filter 22 of FIG. 4 arerespectively expressed as i=I·sin (ωt), v=V·sin (ωt+θ), the aboveequation is represented by the following equation (18).

    a.sub.m =4·I·v·{(sin ((n+1)ωT/2)) /tan (ωT/2)}.sup.2 ·sin (θ)·sin (pωT)

    b.sub.m =4·I.sup.2 ·{sin((n+1)ωT/2)}.sup.2 /tan(ωt/2)·sin(pωT)                  (18)

The operation judgment means 42 is of a structure to make a correctionof reactance setting value X_(s) such that "X_(s) ←X_(s) /tan (ω₀ T/2)"to carry out judgement of operation on the basis of discriminantexpressed as "b_(m) ·X_(a) -a_(m) ≧KO" (KO is sensitivity constant). Theprotective relay of the second embodiment is protective relay havingreactance characteristic similar to FIG. 4, and differs from the systemdisclosed in the first embodiment of FIG. 4 only in the technique forrealization. The reactance characteristic diagram is shown in FIG. 6.

FIG. 7 is a block diagram showing the configuration of the protectiverelay system according to the third embodiment corresponding to thethird aspect.

In the third embodiment, ohm value calculation means 33 for calculatingthe equation (c) is applied as the controlled variable calculation means30.

In accordance with this equation, when current and voltage inputs of thedigital filter 20 consisting of difference filter 21 and addition filter22 of FIG. 4 are respectively expressed as i=I·sin (ωt) and v=V·sin(ωt+θ), -i_(jm) ·v_(sm-p) +v_(sm) ·i_(jm-p) =4·I. ·V·{sin((n+1)ωT/2)}²/t an (ωT/2)·cos (θ)·sin (PωT). Further, from the relationship of theequation (18), the following equation is obtained:

    R.sub.m =(I/V) cos(θ)                                (19)

The operation judgment means 43 is of a structure to judge therelationship in, magnitude between the ohm value calculated by theequation (19) and the setting value R_(s) to judge the protective relayto be operative when "R_(m) ≦R_(s) " holds.

FIG. 8 is a block diagram showing the configuration of the protectiverelay system according to the fourth embodiment corresponding to thefourth aspect of this invention. In the fourth embodiment, ohm valueoperation/suppression quantity calculation means 34 for calculating theequation (D) is applied as the controlled variable calculation means 30.

In the fourth embodiment, when current and voltage inputs of digitalfilter 20 consisting of differential filter 21 and addition filter 22 ofFIG. 3 are respectively expressed as i=I·sin (ωt) and v=V·sin (ωt+θ),c_(m) =i_(jm) ·v_(sm-p) +v_(sm) ·i_(jm-p) =4·I·v·{sin ((n+1)ωT/2) }²/tan (ωT/2)·cos(θ)·sin (PωT). Additionally, b_(m) is the same variableas that in the second embodiment shown in FIG. 5.

The operation judgment means 44 is of a structure to carry out operationjudgment on the basis of b_(m) ·R_(s) -c_(m) ≧k1 from ohm setting valueRs, sensitivity constant K1 and outputs c_(m), b_(m) of theoperation/suppression quantity calculation means 34. The protectiverelay of this embodiment is a protective relay having ohm characteristicin a manner similar to FIG. 5, and differs from the system disclosed inthe second embodiment only in the technique for realization. The ohmcharacteristic diagram is shown in FIG. 9.

FIG. 10 is a block diagram showing the configuration of the protectiverelay system according to the fifth embodiment corresponding to thefifth aspect of this invention. The protective relay of the fifthembodiment is combination of the protective relays which have beenalready described.

Namely, this system comprises controlled variable calculation means 30including reactance value calculation means 31 for carrying out anoperation described below, ##EQU12## which is equivalent to that of thereactance value calculation means 31 in the protective relay system ofthe first embodiment shown in FIG. 4, and ohm value calculation means 33in the third embodiment shown in FIG. 7. Accordingly, output to theoperation judgment means 45 of the controlled variable calculation means30 is reactance value X_(m) and ohm value R_(m).

The operation judgment means 45 judges whether or not the protectiverelay is operative, on the basis of discriminant "(R_(m) -R₀)·(R_(m)-R_(F))+(X_(m) -X₀)·(X_(m) -X_(F))≦0" from output Xm of a reactancevalue calculating means 31A and output Rm of ohm value calculation means33. In this case, R₀, X₀ and R_(s), X_(s) are as indicated below.

offset mho:

near side setting value (R₀ (ohm component) X₀ (reactance component)).

far side setting value (R_(F) (ohm component) , X_(F) (reactancecomponent)).

The protective relay of the fifth embodiment is protective relay havingoffset mho characteristic. The offset mho characteristic diagram isshown in FIG. 11.

The sixth to eighth embodiment of this invention will now be described.These three embodiments respectively correspond to the seventh to ninthaspects, and are conceptually included within the sixth aspect as thehigher rank concept.

Accordingly, the controlled variable calculation means is comprised ofpolarity voltage preparation means for preparing polarity voltagev_(pjm) having a predetermined relationship with respect to thesevoltage data v_(jm) and/or v_(sm) on the basis of the voltage datav_(jm) which is output of the difference filter and voltage data v_(sm)which is output of the addition filter. The predetermined relationshipis somewhat different in the sixth to eighth embodiments.

Moreover, the operation judgment means judges whether or not thefollowing discriminant (F) holds, on the basis of voltage data v_(jm)and current data i_(jm) which are outputs of the difference filter,voltage data vsm and current data i_(sm) which are outputs of theadditive filter, polarity voltage v_(pjm) which is output of thepolarity voltage preparation means, and setting values R_(s) and X_(s).

    v.sub.pjm-p ·{(R.sub.s ·i.sub.sm +X.sub.s ·i.sub.jm)-v.sub.sm }-v.sub.pjm ·{R.sub.s ·i.sub.sm-p +x.sub.s ·i.sub.jm-p)-v.sub.sm-p }≧K2 (F)

FIG. 12 is a block diagram showing the configuration of the protectiverelay system according to the sixth embodiment corresponding to theseventh aspect. The protective relay of the sixth embodiment iscombination of the protective relay already described. The polarityvoltage preparation means 36 extracts voltage variable v_(pjm)orthogonal to output vsm of the addition filter in terms of vector ascontrolled variable calculation means 30. The operation judgment means46 judges, on the basis of the following equation, whether or not,whether or not the protective relay is operative. (R_(s), X_(s)) aresetting values of the ohm component and the reactance component. It isnecessary to use X_(s) after undergone correction of X_(s) ←X_(s)/tan(ω₀ T/2) . In the following equation (F), the electric variable ofthe portion of (R_(s) ·i_(sm) +X_(s) ·i_(m)) leads by magnitude of (R₂ ²+X_(s) ²)^(1/2) and phase φ=tan⁻¹ (X_(s) /R_(s)) with the current i_(sm)being as reference. The relationship thereof is shown in FIG. 13.

    v.sub.pjm-p ·((R.sub.s ·i.sub.sm +X.sub.s ·i.sub.jm)-v.sub.sm) -v.sub.pjm ·((R.sub.s ·i.sub.sm-p +X.sub.s ·i.sub.jm-p)-v.sub.sm-p)≧K2 (F)

When i=I·sin (ωt) and v=V·sin (ωt+θ) are respectively substituted forinput current and input voltage of the difference filter 21 and theaddition filter 22 of the embodiment, the following equation isprovided.

    V.sub.pj ·{(R.sub.s ·I.sub.s ·cos (θ)+X.sub.s /tan (θ.sub.0 T/2)·I.sub.j ·sin (θ))-V.sub.s }·sin (pωT)≧K2

In this case, I_(s), V_(s) and I_(j) are expressed as follows.

I_(s) =2I·sin((n+1)ωT/2)/tan(ωT/2)

V_(s) =2V·sin((n+1)ωT/2)/tan(ωT/2)

I_(j) =2I·sin((n+1)ωT/2)

When the above-mentioned equation is represented by the followingrelational expression is provided. ##EQU13## The above-mentionedequation eventually the equation of the principle of operation of themho characteristic. The mho characteristic is shown in FIG. 14.

FIG. 15 is a block diagram showing the configuration of the protectiverelay system according to the seventh embodiment corresponding to theeighth aspect of this invention. In FIG. 15, operation judgment means 47is provided in place of the operation judgment means 46 shown in FIG.12. In this embodiment, memory voltage earlier by predetermined cycle(data earlier by N samples) of voltage v_(jm) orthogonal to the voltagev_(sm) is caused to be polarity voltage. Namely, ##EQU14## Others aresimilar to those of FIG. 12.

FIG. 16 is a block diagram showing the configuration of the protectiverelay system according to the tenth embodiment corresponding to theninth aspect. In FIG. 16, operation judgment means 48 is provided inplace of the operation judgment means 46 shown in FIG. 12. In thisembodiment, as voltage v_(pjm) orthogonal to the voltage vsm, if thatvoltage is voltage for detection of short circuit, e.g., in the case ofAB phase, positive phase voltage having AB phase as reference isextracted (A, B, C represent respective phases of three-phase a.c.electric variable). The voltage v_(pjm) can be extracted by v_(pjm)(AB)=3^(1/2) ·v_(sm) (c)+v_(jm) (AB) .

Moreover, if positive phase voltage is voltage for grounding, thepositive phase voltage having A phase as reference can be calculated byv_(pjm) (A)=3^(1/2) v_(jm) (A)-v_(jm) (BC) . In addition to the methodin which predetermined (required) voltage vector is synthesized from twoelectric variables orthogonal to each other, a method in which thesampling time series is shifted by 90 degrees may be applied.

FIG. 17 is a block diagram showing the configuration of the protectiverelay system according to the ninth embodiment corresponding to thetenth aspect of this invention. Charge current compensation means 35makes a correction of setting value C_(s) such that C_(s) ←C_(s) /tan(ω₀T/2) to calculate i_(sm) -C_(s) ·v_(jm) from output ism of the additivefilter 22 and output v_(jm) of the differential filter 21. The C_(s)·v_(jm) is compensation component of current produced by the chargecapacity C_(s). The electric variable "(i_(sm) -C_(s) ·v_(jm))B" for thedestination electric station B is received by transmitting/receivingmeans 39, and the electric variable of the terminal for source oftransmission is transmitted to the B electric station.

The operation judgment means 49 carries out operation judgment on thebasis of the following equation from vector sum current of currentobtained by compensating charge current of the terminal for source oftransmission provided by the charge current compensation means 35 andcurrent obtained by compensating charge current of the destinationelectric station B terminal, i.e., scalar sum current of amplitude valueof differential current and amplitude of current obtained bycompensating charge currents of respective terminals. This equation isthe operation principle equation of the ratio differential relay systemwell known as the transmission line differential protective relay

    ∥(i.sub.sm C.sub.s ·v.sub.jm)+(i.sub.sm -C.sub.s ·v.sub.jm B∥≧ka·{∥i.sub.sm -C.sub.s ·v.sub.jm ∥+∥i.sub.sm -C.sub.s ·v.sub.jm)B∥}+kb

where

∥a∥ indicates quantity proportional to amplitude value of a.c. electricvariable a at time point of tm

Ka is No. of ratio suppression digits (absolute number), and

Kb is minimum sensitivity current.

The physical meaning of the charge current compensation of this equationwill be described below with reference to the transmission line of FIG.18. The well known transmission equation is indicated at thetransmitting/receiving terminal.

forward wave: i_(DF) (t)=i_(s) (t-τ)+e_(s) (t-τ)/z +i_(R) (t+τ)-e_(R)(t+τ)/z

backward wave: i_(DB) (t)=i_(s) (t-τ)+e_(s) (t-τ)/z +i_(R) (t+τ)-e_(R)(t+τ)/z

differential current: i_(DD) (t)=(i_(DF) (t)+i_(DB) (t))/2

where suffix s is transmission terminal,

R is charge terminal,

Z is serge impedance=(L/C)^(1/2)

τ is propagation time=1·(LC)^(1/2)

Implementation of Taylor expansion approximation to the differentialcurrent i_(DD) (t) under the condition of (τ≈0) gives: ##EQU15##

If differential current is extracted only by current vector sum current(is(t)+iR(t)) of the transmitting/receiving terminal, the previouslydescribed charge current component would be error current, leading tolowering of sensitivity of the differential relay. Accordingly, sucherror current is compensated so that only the fault current componentcan be extracted. It is apparent as previously described that thisinvention can be applied to the time differential equation of theabove-mentioned equation. In addition, it is omitted that the meaning ofthe ratio differential system is described here.

INDUSTRIAL APPLICABILITY

In accordance with the protective relay system according to thisinvention, even if harmonic components are superimposed on fault currentor fault voltage produced at the time of occurrence of failure of thepower system, an approach is employed to approximate, with higheraccuracy, a predetermined time differential equation by predetermineddigital filters of which characteristics are orthogonal to each other interms of vector in a broad frequency band. Accordingly, there isprovided protective relay system of high accuracy free from influence ofnoise error in failure of the system. Thus, this protective relay systemcan be widely applied to power system including various powerequipments, particularly equipments such as cable transmission line,phase modifying capacitor and the like.

What is claimed is:
 1. A protective relay system adapted for detecting,in time series manner, a voltage variable and a current variable of apower system to discriminate on the basis of changes in the respectivevoltage and current values in the time series whether or not a faultportion of line or equipment included in the power system exists withina predetermined range,the protective relay system comprising:digitalfilter means including a difference filter for outputting voltagedifference variable data indicative of difference between at least twosample data of plural sampling data of the voltage variable and currentdifference variable data indicative of difference between at least twosample data of plural sample data of the current variable at pluralsampling times of the time series, and an addition filter for outputtingvoltage and current additive variable data indicative of datarespectively orthogonal to the voltage and current difference variabledata in terms of vector; and relay control means for calculating areactance value of a relay operation in the power system on the basis ofthe voltage and current difference variable data at a certain samplingtime point, voltage and current additive variable data at the certainsampling time point, the voltage and current difference variable data atany other sampling time point and the voltage and current additivevariable data at the other sampling time point to judge, on the basis ofsaid reactance value, the operation as to whether or not protection ofthe power system should be carried out.
 2. A protective relay systemadapted for detecting, in time series manner, a first electric variableand a second electric variable of a power system to discriminate on thebasis of changes in the respective electric values in the time serieswhether or not a fault portion of line or equipment included in thepower system exists within a predetermined range,the protective relaysystem comprising:digital filter means including a difference filter foroutputting first difference electric variable data indicative ofdifference between at least two sample data of plural sampling data ofthe first electric variable and second difference electric variable dataindicative of difference between at least two sample data of pluralsample data of the second electric variable at plural sampling times ofthe time series, and an addition filter for outputting first and secondadditive electric variable data indicative of data respectivelyorthogonal to the first and second difference electric variable data interms of vector; and relay control means for calculating controlledvariables of a relay operation in the power system on the basis of thefirst and second difference electric variable data at a certain samplingtime point, first and second additive electric variable data at thecertain sampling time point, the first and second difference electricvariable data at any other sampling time point and the first and secondadditive electric variable data at the other sampling time point tojudge, on the basis of the controlled variables, the operation as towhether or not protection of the sower system should be carried out,wherein the difference filter is a filter in which transfer functionZ^(-k) -Z^(-q) (Z is Z transform operator and k<q) is used to outputvoltage data v_(jm) and current data i_(jm) serving as the first andsecond difference electric variable data at a certain sampling timet_(m) and voltage data v_(jm-p) and current data i_(jm-p) at any othersampling time t_(m-p), and the addition filter is a filter in whichtransfer function (1+Z⁻¹ +Z⁻² +. . . +Z^(-n)) (1+Z⁻¹) (where n+1=k+q) isused to output voltage data v_(sm) and current data i_(sm) serving asthe first and second additive electric variable data at the certainsampling time t_(m) and voltage data v_(sm-p) and current data i_(sm-p)at the other sampling time t_(m-p).
 3. A protective relay system as setforth in claim 2,wherein the relay control means includescontrolledvariable calculating means for calculating relay controlled variableincluding at least one of reactance value, resistance value andoperation/suppression quantity on the basis of the voltage data v_(jm)and v_(jm-p) and the current data i_(jm) and i_(jm-p) which are outputsof the difference filter of the digital filter means and the voltagedata v_(sm) and v_(sm-p) and the current data i_(sm) and i_(sm-p) whichare outputs of the addition filter thereof; and operation judging meansfor judging whether or not value of the relay controlled variablecalculated by the controlled variable calculation means has apredetermined relationship with respect to a predetermined setting valueand constant.
 4. A protective relay system as set forth in claim3,wherein the controlled variable calculation means is constituted byreactance value calculation means for determining reactance value X_(m)by the following equation (A) on the basis of the current data i_(jm)and i_(jm-p) which are outputs of the difference filter, and the voltagedata v_(sm) and v_(sm-p) and the current data i_(sm) and i_(sm-p) whichare outputs of the addition filter ##EQU16## wherein the operationjudging means judges the discriminant X_(m) ≦X_(s) from reactance valueX_(m) and setting value X_(S) determined by the reactance calculatingmeans.
 5. A protective relay system as set forth in claim 3,wherein thecontrolled variable calculating means is constituted byoperation/suppression quantity calculating means for calculatingoperation/suppression quantities a_(m) and b_(m) corresponding to thereactance value by the following equation (B) on the basis of thecurrent data i_(jm) and i_(jm-p) which are outputs of the differentialfilter and the voltage data V_(sm) and V_(sm-p) and the current datai_(sm) and i_(sm-p) which are outputs of the additive filter:

    bm=-i.sub.jm ·i.sub.sm-p +i.sub.jm-p ·i.sub.sm (B)

and wherein the operation judging means judges discriminant b_(m) ·X_(s)-a_(m) ≧KO from the operation/suppression quantities a_(m) and b_(m)determined by the operation/suppression quantity calculating means, andsetting value X_(s) and constant KO.
 6. A protective relay system as setforth in claim 3,wherein the controlled variable calculation means isconstituted by ohm value calculating means for determining ohm valueR_(m) by the following equation (C) on the basis of the current datai_(jm) and i_(jm-p) which are outputs of the difference filter and thevoltage data v_(sm) and v_(sm-p) and the current data i_(sm) andi_(sm-p) which are outputs of the addition filter: ##EQU17## and whereinthe operation judgment means judges discriminant R_(m) ≦R_(s) from ohmvalue R_(m) determined by the ohm value calculating means and settingvalue R_(s).
 7. A protective relay system as set forth in claim 3,wherein the controlled variable calculating means is constituted byoperation/suppression quantity calculating means for calculating aquantity corresponding to an ohm value by the following equation (D) onthe basis of the current data i_(jm) and i_(jm-p) which are outputs ofthe difference filter and the voltage data v_(sm) and v_(sm-p) and thecurrent data i_(sm) and i_(sm-p) which are outputs of the additivefilter:

    c.sub.m =-i.sub.jm ·v.sub.sm-p +v.sub.sm ·i.sub.jm-p b.sub.m =-i.sub.jm ·i.sub.sm-p +i.sub.jm-p ·i.sub.sm (D)

and wherein the operation judging means judges discriminant b_(m) ·R_(s)-c_(m) ≦K1 from the operation/suppression quantity c_(m) and b_(m)determined by the operation/suppression quantity calculating means, andsetting value R_(s) and constant K1.
 8. A protective relay system as setforth in claim 3, wherein the controlled variable calculating means isconstituted by reactance value calculating means for determiningreactance value X_(m) by the following equation (A) on the basis of thecurrent data i_(jm) and i_(jm-p) which are outputs of the differencefilter, and the voltage data v_(sm) and v_(sm-p) and the current datai_(sm) and i_(sm-p) which are outputs of the addition filter: ##EQU18##and ohm value calculation means for determining ohm value R_(m) by thefollowing equation (C) on the basis of the current data i_(jm) andi_(jm-p) which are outputs of the difference filter and the voltage datav_(sm) and v_(sm-p) and the current data i_(sm) and i_(sm-p) which areoutputs of the addition filter: ##EQU19## and wherein the operationjudging means serves to discriminate the discriminant (E) on the basisof the reactance value X_(m) determined by the reactance valuecalculating means and the ohm value R_(m) determined by the ohm valuecalculating means:

    (R.sub.m -R.sub.0)·(R.sub.m -R.sub.F)+(X.sub.m -X.sub.0)·(X.sub.m -X.sub.F)≦0            (E)

where R₀ (ohmic component) is setting value at the side close to theoffset mho,X₀ (reactance component) is setting value at the side closeto the offset mho, R_(F) (ohmic component) is setting value at the sideremote from the offset mho, and X_(F) (reactance component) is settingvalue at the side remote from the offset mho.
 9. A protective relaysystem as set forth in claim 3,wherein the controlled variablecalculation means is constituted by polarity voltage preparation meansfor preparing polarity voltage vpjm having a predetermined relationshipwith respect to the voltage data v_(jm) and/or v_(sm) on the basis ofthe voltage data v_(jm) which is output of the difference filter andvoltage data v_(sm) which-is output of the addition filter, and theoperation judgment for judging, on the basis of voltage data v_(jm) andcurrent data i_(jm) which are outputs of the difference filter, voltagedata v_(sm) and current data i_(sm) which are outputs of the additivefilter, polarity voltage V_(pjm) which is output of the polarity voltagepreparation means, and setting values R_(s) and X_(s) : ##EQU20##
 10. Aprotective relay system as set forth in claim 9,wherein the polarityvoltage preparation means synthesizes a voltage orthogonal in afundamental wave with respect to output v_(sm) of the addition filter interms of vector to output the synthesized voltage as the polarityvoltage v_(pjm) to the operation judgment means.
 11. A protective relaysystem as set forth in claim 9,wherein the polarity voltage preparationmeans serves to output a voltage earlier by predetermined time ofvoltage data v_(jm) from the difference filter at the sampling timepoint t_(m) to the operation judgement means.
 12. A protective relaysystem as set forth in claim 9,wherein the polarity voltage preparationmeans serves to synthesize positive phase voltage from output v_(jm) ofthe difference filter and output v_(sm) of the additive filter withthree-phase voltage of the power system at the sampling time t_(m) beingas reference to output the synthesized voltage as the polarity voltagev_(pjm) to the operation determination means.
 13. A protective relaysystem as set forth in claim 3,wherein the controlled variablecalculation means is composed of:charge current compensation means fordetermining predetermined electric variable (i_(sm) -C_(s) ·v_(jm)) onthe basis of the voltage data v_(jm) as output of the differentialfilter at the sampling time point t_(m), the current data i_(sm) asoutput of the additive filter, and setting value C_(s) ; andtransmitting/receiving means adapted for transmitting the predeterminedelectric variable (i_(sm) -C_(s) ·v_(jm)) delivered from the chargecurrent compensation means to a destination electric station except forthe electric station where the protective relay system is installed andfor receiving electric variable (i_(sm) -C_(s) ˜v_(jm)) B from any otherprotective relay system installed in the destination electric station,and wherein the operation judgment means serves to judges, on the basisof the predetermined electric variable (i_(sm) -C_(s) ·v_(jm)) deliveredfrom the charge current compensation means and the predeterminedelectric variable (i_(sm) -C_(s) ·v_(jm)) B delivered from thetransmitting/receiving means whether or not the following discriminant(G) holds:

    ∥(i.sub.sm -C.sub.s ·v.sub.jm)+(i.sub.sm -C.sub.s ·v.sub.jm)B∥≧ka{∥i.sub.sm -C.sub.s ·v.sub.jm ∥+∥(i.sub.sm -C.sub.s ·v.sub.jm)B∥}+kb                        (G)

where∥a∥ is quantity proportional to amplitude value of a.c. electricvariable a at the time point of t_(m), ka is No. ratio suppressiondigits (absolute number), and kb is minimum sensitivity current.